Rail vehicle bodies are already known that are made of stainless steel and that are constituted by stainless steel sheets welded onto a strength-providing structure. Unpainted stainless steel has the property of being easy to clean, and this is a considerable advantage in the maintenance of suburban trains that are often subject to graffiti.
For reasons of appearance, and given the large amount of deformation that occurs due to heat, it is necessary to take certain precautions when welding very thin sheets of stainless steel on the strength-providing structure of the body. The sheet metal is then fixed to the strength-providing structure by the resistance spot welding method. The procedure is as follows. Pieces of sheet metal are placed against the strength-providing structure so as to overlie the length members of the chassis and the roof battens, and they are fastened to the structure by spot welds. Wherever sealing is required, an electroplastic sealant is interposed between the parts to be welded together.
The sheet metal is first heated and then strained by mechanical tension prior to being welded onto the strength providing structure.
Although resistance spot welding is satisfactory from the appearance point of view for an unpainted stainless steel vehicle, it is insufficient when applied to the strength-providing structures of the body (length members, roof battens, door uprights) with respect to guaranteeing integrity of the passenger volume in the event of a collision. In addition, market forces are leading to smooth-faced designs and ever tighter targets concerning mass savings, productivity, and ease of manufacture in order to enable the technology to be exported.
The above lead to the use of covering sheets on the smooth faces that are as thin as possible and to their being fixed to the structure by spots which are associated with weld fillets (requiring large amounts of energy) so as to cause the metal sheets to contribute to the strength of the body. Under such conditions, and with a rail vehicle of the known prior art type as described above, it is necessary to have recourse to operations for making the metal sheets plane (e.g. by beating that leave traces that are highly visible). Such operations are difficult given the appearance of unpainted metal sheets.